fix: Project-scoped job claiming, structured rate-limit logging, RRF total_cmp

Targeted fixes across multiple subsystems:

dependent_queue:
- Add project_id parameter to claim_jobs() for project-scoped job claiming,
  preventing cross-project job theft during concurrent multi-project ingestion
- Add project_id parameter to count_pending_jobs() with optional scoping
  (None returns global counts, Some(pid) returns per-project counts)

gitlab/client:
- Downgrade rate-limit log from warn to info (429s are expected operational
  behavior, not warnings) and add structured fields (path, status_code)
  for better log filtering and aggregation

gitlab/transformers/discussion:
- Add tracing::warn on invalid timestamp parse instead of silent fallback
  to epoch 0, making data quality issues visible in logs

ingestion/merge_requests:
- Remove duplicate doc comment on upsert_label_tx

search/rrf:
- Replace partial_cmp().unwrap_or() with total_cmp() for f64 sorting,
  eliminating the NaN edge case entirely (total_cmp treats NaN consistently)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ingestion/merge_requests.rs

@@ -314,7 +314,6 @@ fn process_mr_in_transaction(
     })
 }
 
-/// Upsert a label within a transaction, returning its ID.
 /// Upsert a label within a transaction, returning its ID.
 /// Uses INSERT...ON CONFLICT...RETURNING for a single round-trip.
 fn upsert_label_tx(